Silicon based nanospheres and nanowires

ABSTRACT

A nanowire, nanosphere, metallized nanosphere, and methods for their fabrication are outlined. The method of fabricating nanowires includes fabricating the nanowire under thermal and non-catalytic conditions. The nanowires can at least be fabricated from metals, metal oxides, metalloids, and metalloid oxides. In addition, the method of fabricating nanospheres includes fabricating nanospheres that are substantially monodisperse. Further, the nanospheres are fabricated under thermal and non-catalytic conditions. Like the nanowires, the nanospheres can at least be fabricated from metals, metal oxides, metalloids, and metalloid oxides. In addition, the nanospheres can be metallized to form metallized nanospheres that are capable as acting as a catalyst.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to copending U.S. Provisionalapplication entitled, “Silicon Based Nanowires and Nanospheres”, filedwith the United States Patent and Trademark Office on Mar. 29, 2000, andassigned Serial No. 60/192,846, and U.S. provisional applicationentitled “New Cu/SiO₂ Based Catalyst for Selective Ethanol-AcetaldehydeConversion”, filed with the United States Patent and Trademark Office onMar. 29, 2000, and assigned Serial No. 60/192,844, which are bothentirely incorporated herein by reference.

TECHNICAL FIELD

[0002] The present invention is generally related to nanostructures and,more particularly, is related to nanowires and nanospheres and methodsfor their preparation and use.

BACKGROUND OF THE INVENTION

[0003] Semiconductor nanostructures, nanoagglomerates, and nanowireshave attracted considerable attention because of their potentialapplications in mesoscopic research, the development of nanodevices, andthe potential application of large surface area structures. For severaldecades, the vapor-liquid-solid (VLS) process, where gold particles actas a mediating solvent on a silicon substrate forming a molten alloy,has been applied to the generation of silicon whiskers. The diameter ofthe whisker is established by the diameter of the liquid alloy dropletat its tip. The VLS reaction generally leads to the growth of siliconwhiskers epitaxially in the <111> direction on single crystal silicon<111> substrates. In addition, laser ablation techniques have beenperformed on metal-containing (iron or gold) silicon targets, producingbulk quantities of silicon nanowires. Further, thermal techniques havebeen used to produce a jumble of silicon dioxide (SiO₂) coatedcrystalline nanowires that have their axes parallel to the <112>direction. Further these nanowires are deficient because of twinning,high order grain boundaries, and stacking faults.

[0004] Recently, national lab researchers, in an effort to begin anongoing dialogue to forecast the direction of environmental science andtechnology, ranked the top ten environmental technology breakthroughsfor 2008. Not surprisingly, molecular design is expected to play animportant role in the development of advanced materials. Included inthis framework is the design of nano-assembled and non-stoichiometriccatalysts designed for the efficient control of chemical processes.

[0005] Heterogeneous catalysts are typically prepared by decorating highsurface area solids such as silica or alumina with active metals ormetal ions from precursor materials such as cation complexes[M^(n+)(L^(m−) _(x)]^((n−xm)), anion complexes (e.g., [Pt⁴⁺F₆]²⁻ orneutrals such as copper (II) acetylacetonate (Cu(AcAc)₂)). Theseprocesses typically use starting reagents and produce products that areharmful to the environment (e.g. solvents, metal halides, strong acids,or other environmentally aggressive reagents and or products). Ahigh-surface-area support is needed to provide the proper dispersion ofthe active ingredients so that the high intrinsic activity of thesecatalytic metals or ions can be realized in practice. Without thissupport, many catalytic agents show very little active surface area.Often, the intrinsic catalytic activity of the supported metals or metalions is changed by interaction with the support metal ions or oxygenatoms. Thus, some supports are not benign towards the catalytic agents.Moreover, the catalytic properties of these agents are often compromisedas a result of the efforts to synthesize supported catalysts having highdispersions of the active ingredient. These uniquely assembled catalystsmight then be used to more efficiently control combustion processes, andreactions such as hydrocarbon reforming.

[0006] Thus, a heretofore unaddressed need exists in the industry toaddress the aforementioned deficiencies and inadequacies.

[0007] An embodiment of the present invention provides for a nanowireand method of fabrication thereof. The method includes fabricating thenanowires under thermal and non-catalytic conditions. The nanowires canbe fabricated from at least metals, metal oxides, metalloids, andmetalloid oxides. A preferred embodiment of the present inventionincludes, but is not limited to, the fabrication of a silicon dioxidesheathed crystalline silicon nanowire, where the axis of the crystallinesilicon nanowire core is substantially parallel to a <111> plane and issubstantially free of defects.

[0008] Another embodiment of the present invention provides for ananosphere and method of fabrication thereof. The method includesfabricating the substantially monodisperse nanospheres under thermal andnon-catalytic conditions. The nanospheres can at least be fabricatedfrom metals, metal oxides, metalloids, and metalloid oxides. A preferredembodiment of the present invention includes, but is not limited to,fabricating amorphous silicon dioxide nanospheres.

[0009] Still another embodiment of the present invention provides for ametallized nanosphere and method of fabrication thereof. The methodincludes fabricating the subtantially monodisperse nanospheres underthermal and non-catalytic conditions. The nanospheres can be fabricatedfrom at least metals, metal oxides, metalloids, and metalloid oxides.The nanospheres can be metallized to form metallized nanospheres thatare capable of having catalytic properties. In addition, the formationof the nanospheres and metallization of the nanospheres can be performedsubstantially in one step. A preferred embodiment of the presentinvention includes fabricating amorphous silicon dioxide nanospheres anddepositing three weight percent (%) copper onto the nanosphere.

[0010] Still a further embodiment of the present invention provides fora method of the dehydrogenation of ethanol. The method includesintroducing gaseous ethanol to three weight percent metallized silicondioxide nanospheres to produce at least a three percent conversion/mgcopper for the selective dehydrogenation of ethanol into acetaldehyde.

[0011] Other systems, methods, features, and advantages of the presentinvention will be or become apparent to one with skill in the art uponexamination of the following drawings and detailed description. It isintended that all such additional systems, methods, features, andadvantages be included within this description; be within the scope ofthe present invention, and be protected by the accompanying claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0012] Embodiments of the present invention provide for nanostructures,catalytic nanostructures, and methods of preparation of same.Nanostructures include, but are not limited to, nanowires, nanospheres,nanoagglomerates, nanotubes, etc. More specifically, exemplaryembodiments of the present invention provide a nanowire and methods ofpreparation thereof. Another exemplary embodiment provides a nanosphereand methods of preparation thereof. Still another exemplary embodimentprovides a catalytic nanosphere and methods of preparation thereof(e.g., a metallized nanosphere with catalytic activity). Thenanostructures can be made of materials such as, but not limited to,metals, metal oxides, metalloids, metalloid oxides, combinations ofmetals, combinations of metal oxides, combinations of metalloids,combinations of metalloid oxides, combinations of metals and metaloxides, combinations of metalloid and metalloid oxides, or any otherappropriate combination. Further, the nanostructures can be metallizedto form catalytic nanostructures that can be used to enhance reactionkinetics and reaction efficiency.

[0013] A. Nanowires and Nanospheres

[0014] One exemplary embodiment of the present invention provides for ananowire prepared under thermal and non-catalytic conditions. Thethermal conditions include, but are not limited to, the range of 800° C.to 1500° C. The term non-catalytic conditions means, for the purposes ofthis disclosure, that an additional catalyst is unnecessary for thenanostructures to be fabricated. In an exemplary embodiment, thenanowire can be fabricated to form metal, metal oxide, metalloid,metalloid oxide, or combinations thereof nanowires. In a preferredembodiment, the nanowires include silicon dioxide sheathed crystallinesilicon nanowires where the axis of the crystalline silicon nanowirecore is substantially parallel to a <111> plane. In addition, thesilicon nanowires are substantially defect free. That is, the siliconnanowires are substantially free of twinning, high order grainboundaries, and stacking faults. Non-limiting examples of metals fromwhich the nanowires can be fabricated include, but are not limited to,tin (Sn), chromium (Cr), iron (Fe), nickel (Ni), silver (Ag), titanium(Ti), cobalt (Co), zinc (Zn), platinum (Pt), palladium (Pd), osmium(Os), gold (Au), lead (Pb), iridium (Ir), molybdenum (Mo), vanadium (V),aluminum (Al), or combinations thereof. In addition, non-limitingexamples of metal oxides which the nanowires can be fabricated intoinclude, but not limited to, tin dioxide (SnO₂), chromia (Cr₂O₃), ironoxide (Fe₂O₃, Fe₃O₄, or FeO) nickel oxide (NiO), silver oxide (AgO),titanium oxide (TiO₂), cobalt oxide (Co₂O₃, Co₃O₄, or CoO), zinc oxide(ZnO), platinum oxide (PtO), palladium oxide (PdO), vanadium oxide(VO₂), molybdenum oxide (MoO₂), lead oxide (PbO), and combinationsthereof. In addition, a non-limiting example of a metalloid includes,but is not limited to, silicon or germanium. Further, a non-limitingexample of a metalloid oxide includes, but is not limited to, siliconmonoxide, silicon dioxide, germanium monoxide, and germanium dioxide.

[0015] Another exemplary embodiment of the present invention providesfor a plurality of nanospheres that are substantially monodisperse and amethod of preparation thereof. In addition, the nanospheres can befabricated in gram quantities under thermal and non-catalyticconditions. The thermal condition includes, but is not limited to, therange of 800° C. to 1500° C. The term non-catalytic conditions meansthat an additional catalyst is unnecessary for the nanostructures to befabricated. Further, the nanospheres can be fabricated to form metal,metal oxide, metalloid, metalloid oxide, or combinations thereofnanospheres. Non-limiting examples of metals from which the nanospherescan be fabricated include, but are not limited to, tin (Sn), chromium(Cr), iron (Fe), nickel (Ni), silver (Ag), titanium (Ti), cobalt (Co),zinc (Zn), platinum (Pt), palladium (Pd), osmium (Os), gold (Au), lead(Pb), iridium (Ir), molybdenum (Mo), vanadium (V), aluminum (Al), andcombinations thereof. In addition, non-limiting examples of metal oxidesfrom which the nanospheres can be fabricated include, but not limitedto, tin dioxide (SnO₂), chromia (Cr₂O₃), iron oxide (Fe₂O₃, Fe₃O₄, orFeO), nickel oxide (NiO), silver oxide (AgO), titanium oxide (TiO₂),cobalt oxide (Co₂O₃, Co₃O₄, or CoO), zinc oxide (ZnO), platinum oxide(PtO), palladium oxide (PdO), vanadium oxide (VO₂), molybdenum oxide(MoO₂), lead oxide (PbO), and combinations thereof. In addition, anon-limiting example of a metalloid includes, but is not limited to,silicon and germanium. Further, a non-limiting example of a metalloidoxide includes, but is not limited to, silicon monoxide, silicondioxide, germanium monoxide, and germanium dioxide. The nanospheres canrange in diameter from a few nanometers to on the order of hundreds ofnanometers. More particularly, silicon dioxide nanospheres areamorphous, have no dangling bonds, and range in diameter from about 8-45nanometers (run). Further, the method of fabricating nanospheres andnanowires using thermal techniques can be similar. In this regard, bothnanospheres and nanowires can be fabricated using similar fabricationsteps. Modifications in fabrication parameters, disclosed hereinafter,can be used to control the quality and quantity of the fabricatednanospheres and nanowires.

EXAMPLE 1

[0016] The following is a non-limiting illustrative example of anembodiment of the present invention that is described in more detail inGole, et al., Appl. Phys. Lett., 76, 2346 (2000), which is incorporatedherein by reference. This example is not intended to limit the scope ofany embodiment of the present invention, but rather is intended toprovide specific experimental conditions and results. Therefore, oneskilled in the art would understand that many experimental conditionscan be modified, but it is intended that these modification are withinthe scope of the embodiments of the present invention.

[0017] The apparatus to fabricate silicon based nanostructures includesa double concentric alumina tube combination that can be heated to thedesired temperature in a Lindberg Scientific tube furnace configuration.The inner alumina tube is vacuum sealed by two water cooled stainlesssteel end pieces which are attached to the alumina tube and tightlylock-press fit against custom viton o-rings. At one end of the furnace,ultra-high purity argon (Ar) enters through the upstream stainless steelend piece and passes through a matched set of zirconia insulators to thecentral region of the inner tube oven. Here the entraining argon flowsover a crucible containing the sample mixture of interest, which may beeither a silicon-silica (Si/SiO₂) mixture or powdered silicon monoxide,at a flow rate of 100 standard cubic centimeter per minute (sccm)controlled by a flow controller. It should be noted that other samplemixtures can be used that correspond to the metals listed hereinabove.

[0018] The total tube pressure in the inner tube can range from 200 to650 Torr as measured by a Baratron differential pressure transducer, butis typically about 225 Torr. The pressure in the inner tube can becontrolled by a mechanical pump or other appropriate pump attached tothe inner alumina tube through the downstream stainless steel end piece.This end piece is mechanically attached to a “water cooled” cold plate,with an adjustable temperature system, through a matching set ofinsulating zirconia blocks. Depending on the desired temperature rangeof operation, the crucibles used to contain the silicon/silicon oxidebased mixtures were either commercially available quartz (1200-1350° C.)or alumina (1400-1500° C.) or were machined from low porosity carbon(1500° C.). The parameters that can be controlled in this experimentwere (1) gas flow rate, (2) total tube gas pressure, (3) central regiontemperature and temperature gradients to the end regions, and (4) coldplate temperature. The ultra-high purity argon was not heated before itenters the inner furnace tube, although it could be heated. Thecondensation of silicon-based nanowires produced dark brown deposits ina narrow region on the wall of the inner alumina tube, close to thedefining end points of the Lindberg oven shell, which corresponds to atemperature in the range approximately 900-1000° C. Large quantities(e.g. gram quantities) of SiO₂ nanospheres were deposited on thetemperature controlled cold plate.

[0019] In an exemplary embodiment, virtually uniform and straightnanowires were generated from a 50/50 Si/SiO₂ equimolar mixture heatedto a temperature of about 1400° C. at a total pressure of about 225 Torrfor about 12 hours. The central crystalline silicon core for thenanowire is about 30 nm in diameter, whereas the outer SiO₂ sheathing isabout 15 nm in thickness, as exemplified in Gole et al., Appl. Phys.Lett. 76, 2346 (2000), which is incorporated herein by reference.However, nanowires with much smaller and larger diameter centralcrystalline cores and different sheathing thickness have been obtained.The axis of the SiO₂ clad crystalline silicon nanowire core issubstantially parallel to the <111> plane. This is distinct from theresults obtained by Lee et al.; MRS Bulletin, 36 (1999) whose wires havetheir axis parallel to <112> plane as they display twinning, high ordergrain boundaries, and stacking faults. At the Si—SiO₂ interface for thematerial obtained in the present synthesis the crystal planes are bestdescribed as {211}. The nanowires synthesized are so perfect that slightundulations of the crystalline silicon core, due to strain induced bymeasuring devices, can be observed.

[0020] Other distinguishing characteristics of the nanowires include thepinch off of the crystalline silicon core at the beginning of the wiregrowth, suggesting a distinctly different formation mechanism than thatsuggested by Lee et al. for their wires generated using a similar sourceand by Hu et al., Acc. Chem. Res. 32, 435 (1999) for theiriron-catalyzed wire formation from Fe/Si mixtures generated using laserablation. While Lee et al. find evidence for a growth mechanism along<111> with which they associate a complex process involving SiO₂formation, the observed structures generated using the described thermalsource likely indicate that the mechanism for these nanowires is a closeanalogy to the VLS mechanism, albeit with an apparent self-assembly ofthe silicon in the absence of a metal catalyst. Further, the outer SiO₂sheath of the nanowire has significant strength. Finally, a comparisonto the transmission electron micrograph (TEM) micrographs of Hu et al.,which show the clear termination of their nanowires at larger —nearlyspherical FeSi₂ nanoclusters, offers yet an additional contrastsuggesting further alternate mechanisms for the wire formation. Themechanism for formation of the nanowires in the present study wouldappear to be distinct and possess both the attributes of the Si/SiO₂reaction mechanism presented by Lee et al. and of the VLS growth method.

[0021] Nearly monodisperse SiO₂ nanospheres in the diameter range of8-45 nm can be generated as a deposit in gram quantities on the coldplate of the described apparatus. Nanospheres can be generated in thesame apparatus that produced the nanowires. By adjusting the flowparameters and temperature, it is possible to generate nanospheresranging in diameter from 8-45 nm in virtually monodispersedistributions. It is possible to generate these nanospheres not onlyfrom Si/SiO₂ mixtures but also from SiO powders, albeit at somewhathigher temperatures.

[0022] Judicious manipulation of the high temperature system includingreactant mixture stoichemistry, flow conditions (kinetics), andtemperature range, may yield more than would have been previouslyanticipated by others skilled in the art. The results suggest thatadditional mechanisms which are analogs not only of the VLS mechanism onthe nanoscale but also represent some crystalline silicon self-assemblymay be operative. Further, Lee et al. produce a jumble of uniform SiO₂coated crystalline silicone nanowires of various sizes which, whenstraight, have their axes parallel to <111>. These wires, however,display twining, high order grain boundaries, and defect sites (stackingfaults). In contrast, embodiments of the present invention are capableof producing nanowires where the axis of the nanowire core issubstantially parallel to a <111> plane, virtually defect free, anddemonstrate no twining. Given the high temperature synthesis ofalternate combinations of metal/metal oxide nanowire configurations,embodiments of the present invention appear to be well suited tophotonic waveguide applications.

[0023] B. Nanosphere Catalysts

[0024] Still another exemplary embodiment of the present inventionprovides a catalytic nanosphere (e.g., metallized nanosphere) and methodof preparation thereof. The nanosphere of this embodiment can be formedin a manner similar to the preparation of nanospheres described earlierand includes the same properties as those nanospheres. After thenanospheres are fabricated, the nanospheres can be metallized to formmetallized nanospheres that are capable of having catalytic properties.One of many advantages of this embodiment is that the nanosphere andmetallized nanosphere can be fabricated in one step rather than multiplesteps, as required by present techniques in the art.

[0025] Non-limiting examples of metals from which the nanospheres can befabricated include, but are not limited to, tin (Sn), chromium (Cr),iron (Fe), nickel (Ni), silver (Ag), titanium (Ti), cobalt (Co), zinc(Zn), platinum (Pt), palladium (Pd), osmium (Os), gold (Au), lead (Pb),iridium (Ir), molybdenum (Mo), vanadium (V), aluminum (Al), andcombinations thereof. In addition, non-limiting examples of metal oxidesfrom which the nanospheres can be fabricated include, but not limitedto, tin dioxide (SnO₂), chromia (Cr₂O₃), iron oxide (Fe₂O₃, Fe₃O₄, orFeO), nickel oxide (NiO), silver oxide (AgO), titanium oxide (TiO₂),cobalt oxide (Co₂O₃, Co₃O₄, or CoO), zinc oxide (ZnO), platinum oxide(PtO), palladium oxide (PdO), vanadium oxide (VO₂), molybdenum oxide(MoO₃), lead oxide (PbO), and combinations thereof. In addition, anon-limiting example of a metalloid includes, but is not limited to,silicon and germanium. Further, a non-limiting example of a metalloidoxide includes, but is not limited to, silicon monoxide, silicondioxide, germanium monoxide, and germanium dioxide. The nanospheres canrange in diameter. More particularly, silicon dioxide nanospheres areamorphous, have no dangling bonds, and range in diameter from about 8-45nanometers.

[0026] Further, the method of metallization is capable of depositing asecond metal onto the nanosphere. The term “second metal” is used hereto differentiate the material (e.g. metal, metalloid, or oxides thereof)that the nanosphere may be fabricated into, and refers to the metal thatis deposited upon the nanosphere during a metallization process. Thesecond metals that can be deposited during the metallization processinclude, but are not limited to, copper, tin, aluminum, silver,platinum, palladium, iron, cobalt, nickel, combinations thereof, andother appropriate metallization metals.

EXAMPLE 2

[0027] The following is a non-limiting illustrative example of anembodiment of the present invention that is described in more detail inGole, et al., submitted to J. Appl. Phys., Gole et al. submitted toChemistry of Materials, which are herein incorporated by reference. Thisexample is not intended to limit the scope of any embodiment of thepresent invention, but rather is intended to provide specificexperimental conditions and results. Therefore, one skilled in the artwould understand that many experimental conditions can be modified, butit is intended that these modifications are within the scope of theembodiments of the present invention.

[0028] Silica nanospheres, of about a 30 nm diameter, can be prepared atelevated temperature (e.g. 800-1500° C.) from an Si/SiO₂ mixture. Underambient conditions, the high population of surface hydroxyl groups onthese nanospheres, confirmed by FTIR spectroscopy, is probed bydecorating the surfaces of the spheres with the metal complex copper(II) acetylacetonate: Cu(AcAc)₂. These metal complexes are known in theart to be anchored by the surface SiOH species, and can be convertedinto an active catalyst by thermolysis of the ligands. The resultingmonatomic copper distribution forms a selective catalyst whoseconversion efficiency appears to be at least comparable to, if notbetter than, Cu/fumed silica described in Kenvin, et al., J. Catal. 135,81 (1992). In contrast to the fumed silica, however, the preparation ofthis catalyst support is environmentally benign.

[0029] Dispersed nanospheres have been fabricated without the use ofsolvents and without producing byproducts, such as hydrochloric acidgas, to compromise the environment. The synthesis technique uses amixture of silicon and silicon dioxide, heated under a flow of ultrahigh purity argon at elevated temperature for a specified duration. Thesynthesis method can produce silica nanospheres, having nearlymonodisperse particle size of about 30 nm. These nanospheres, asdemonstrated by high-resolution transmission electron microscopy andx-ray diffraction, are amorphous. Further, as elaborated in more detailin this example, the silica nanosphere has surface properties thatdemonstrate the presence of surface silanol groups (—SiOH) which can beused to sequester active Cu sites for the selective conversion ofethanol to acetaldehyde. A surface population of —SiOH groups on silicacan influence the bonding of metal complexes to the surface. The loadingof the metal complexes and the resulting morphology of the supportedmetal ions is influenced by the —SiOH groups on the surface. Silicananospheres are contacted with Cu(AcAc)₂ in acetonitrile in sufficientconcentration to produce silica nanospheres that contain about 3 wt %Cu. This same procedure has been used to make monatomic dispersions ofCu ions on fumed amorphous silica manufactured and commerciallyavailable from the Cabot Corporation (Cab-O-Sil™) Alpharetta, Ga.

[0030] The products of the ethanol dehydrogenation reaction depend uponthe ensemble size of supported Cu ions. Isolated copper ions catalyzeonly the dehydrogenation to acetaldehyde whereas multiple Cu ensemblesshow high yields of ethyl acetate in addition to acetaldehyde. Thus, theethanol/acetaldehyde probe reaction can be used to define the presenceof monatomic dispersions of Cu ion from an examination of the productdistribution.

[0031] TEM micrographs indicate that nearly monodisperse SiO₂nanospheres of diameter of close 30 nm can be generated in gramquantities on the cold plate of the high temperature synthesis devicedescribed earlier. As described earlier, the apparatus includes a doubleconcentric alumina tube combination heated to the desired temperature ina Lindberg Scientific tube furnace configuration. The inner alumina tubeis vacuum sealed by two water cooled stainless steel end pieces whichare attached to the alumina tube and tightly lock-press fit againstcustom viton™ o-rings. At one end of the furnace, ultra high purityargon enters thru the upstream stainless steel end piece and passesthrough a matched set of zirconia insulators to the central region ofthe inner tube oven. The entraining argon then flows over a cruciblecontaining the sample mixture of interest, which is either asilicon-silica (Si/SiO₂) mixture or powdered silicon monoxide, at a flowrate of 100 sccm controlled a flow controller.

[0032] The total tube pressure in the inner tube can range from 200 to650 Torr but is typically about 225 Torr. This pressure can becontrolled by a mechanical pump or other appropriate pump attached tothe inner alumina tube through the downstream stainless steel end piece.This end piece is mechanically attached to a water cooled cold plate,which has as adjustable temperature system, through a matching set ofinsulating zirconia blocks. Depending on the desired temperature rangeof operation, the crucibles used to contain the silicon/silicon oxidebased mixtures are either commercially available quartz (1200-1350° C.)or alumina (1400-1500° C.) or are machined from low porosity carbon(1500° C.). The controlled parameters may include for example, but notlimited to, (1) gas flow rate, (2) total tube gas pressure, (3) centralregion temperature and temperature gradients to the end regions, and (4)cold plate temperature. It is to be noted that, at least for theexperimental results reported here, no attempt was made to heat theultra high purity argon before it enters the inner furnace tube. Largequantities of SiO₂ nanospheres were deposited on the temperaturecontrolled cold plate.

[0033] The Cu/silica catalysts were prepared through batch impregnationof 1 g of the silica with sufficient Cu(AcAc)₂ metal complex to producea sample having 3 wt % Cu. The complex was added to 25 mL ofacetonitrile solvent and allowed to reflux with stirring for 24 h. Thesolid was separated by filtration and dried at room temperature for 18h. This solid was dried at 100° C. for 1 hour then placed in amicroreactor tube.

[0034] The ethanol dehydrogenation reaction was completed in amicro-catalytic reactor. Prior to the reaction, the nanosphere catalystwas heated to about 350° C. for about 1 h in flowing helium, then cooledto the reaction temperature. The reaction conditions were conducted atabout 330° C., 20 mL per minute of He carrier gas flow over a 100 mg bedof catalyst having a Cu loading of 3 weight percent. Five to term pLpulses of ethanol were vaporized into the He carrier gas stream tocreate the reactant feed. Pulses of unreacted ethanol and the productsof reaction were partitioned on a GC column and detected by a thermalconductivity detector.

[0035] The silica nanospheres have been characterized by FourierTransform Infrared (FTIR) spectroscopy. The nanospheres were scannedjust after their introduction into the sample chamber at 25° C. and 1atm. Subsequently, the samples were evacuated to <I milli-Torr at 25° C.and their spectrum was recorded. The nanospheres were then heated to100, 200, and 300° C. in vacuo and their spectra were recorded underthese conditions.

[0036] Under 1 atm pressure at room temperature, the sample shows alarge, broad peak between 3000 and 4000 cm⁻¹ that is characteristic ofadsorbed, molecular water. This feature decreases to a negligible levelimmediately upon evacuation at room temperature. This result suggeststhat most of the water is only weakly adsorbed to the silicananospheres. Additional peaks are present at 1800, 1600, 1200 and 800cm⁻¹. In vacuo at 25 C, a sharp peak appears at 3700 cm⁻¹ and a broadpeak near 3400 cm⁻¹. When the sample is heated to 300 C under vacuum,the peak at 3700 cm⁻¹ grows even sharper and the adjacent peak at 3400cm⁻¹ grows smaller, demonstrating further water removal. With increasedheating above 200 C, the peaks at 1200 and 800 cm⁻¹ at first increaseand then broaden and decrease in intensity as a shift of intensity tohigher frequency features is apparent.

[0037] Flame-hydrolyzed, amorphous silica shows a signature for theSiO—H vibration near 3743 cm⁻¹ and a broad peak near 3400 cm⁻¹ thatcorresponds to adsorbed water. Additionally. Si—O vibrations are evidentat 1800 and 1600 cm⁻¹. It appears that the surface functional groupsfound on the silica nanospheres are similar to those found to be presenton Cab-O-Sil™.

[0038] The effect of the Cu/silica nanocatalyst on the ethanoldehydrogenation reaction is presented in Table 1. Acetaldehyde was theonly product observed. Forty five percent of the ethanol was convertedover about 3 mg of Cu in the 100 mg sample of Cu/silica using thenano-silica sample. The conversion per mg of Cu in this sample is 45%/3mg or 15% conversion/mg Cu. Compare this to the results reported byKenvin et al. for a Cu/silica prepared from Cab-O-Sil™ and operatedunder similar conditions (300 C, 5.1, mg Cu ion +143 mg of silica, 15.5mL/minute of He carrier, 1-2 L of ethanol in liquid pulses). Theseauthors observed 25% conversion over 5.1, mg Cu for a 5.1% conversion/mgCu. No other products were observed.

[0039] These results demonstrate that the conversion efficiency for thecatalyst formed from the copper loaded silica nanospheres is at leastcomparable to if not better than that formed from the fumed silica(within the accuracy of the micro-catalytic technique for determiningcatalyst activity). Moreover, the selectivity to form acetaldehyde isthe same for the two catalysts. Each solid catalyzes the single reactionto form the simple dehydrogenation product without the side reactioncorresponding to ethyl acetate coupling. The absence of the ethylacetate forming reaction shows that no large ensembles of Cu are presentin either sample. TABLE 1 SUMMARY OF RESULTS Nanosphere Fused SilicaSpecies mol % mol % EtOH 55 75 Acetaldehyde 45 25 Other products 0 0

[0040] The results obtained using the nanospheres clearly demonstratethat only the products of mono-atomically dispersed Cu (onlyacetaldehyde is observed) with an apparently improved efficiency. Itshould be noted that the process for forming the new catalyst suggestsan additional advantage in that it might replace the present techniquefor making fumed amorphous silica by a process that is environmentallybenign. The currently applied process for making fumed silica burnssilicon tetrachloride to make silica and HCl. The present embodiment,which relies on an elevated temperature synthesis involving only anSi/SiO₂ mixture, eliminates the need to handle silicon tetrachloride andit does not produce the hydrochloric acid gas.

[0041] It should be emphasized that the above-described embodiments ofthe present invention, particularly, any “preferred” embodiments, aremerely possible examples of implementations, merely set forth for aclear understanding of the principles of the invention. Many variationsand modifications may be made to the above-described embodiment(s) ofthe invention without departing substantially from the spirit andprinciples of the invention. All such modifications and variations areintended to be included herein within the scope of this disclosure andthe present invention and protected by the following claims.

Therefore, having thus described the invention, at least the followingis claimed:
 1. A method of preparing a nanostructure, comprising thestep of forming a nanowire under thermal conditions and undernon-catalytic conditions.
 2. The method of claim 1, wherein the step offorming the nanowire under thermal conditions comprises the step offorming a nanowire in the temperature range of about 800° C. to about1500° C.
 3. The method of claim 1, wherein the step of forming thenanowire comprises the step of forming a metal nanowire.
 4. The methodof claim 3, wherein the step of forming the metal nanowire, comprisesthe step of forming a metal nanowire, wherein the metal is selected fromthe group consisting of: tin, chromium, iron, nickel, silver, titanium,cobalt, zinc, platinum, palladium, osmium, gold, lead, iridium,molybdenum, vanadium, and aluminum.
 5. The method of claim 3, whereinthe step of forming the metal nanowire, comprises the step of forming ametal oxide nanowire, wherein the metal oxide is selected from the groupconsisting of: tin dioxide, chromia, iron oxide, nickel oxide, silveroxide, titanium oxide, cobalt oxide, zinc oxide, platinum oxide,palladium oxide, vanadium oxide, molybdenum oxide, and lead oxide. 6.The method of claim 1, wherein the step of forming the nanowirecomprises the step of forming a metalloid nanowire.
 7. The method ofclaim 6, wherein the step of forming the metalloid nanowire, comprisesthe step of forming a silicon dioxide sheathed crystalline siliconnanowire, where the axis of the crystalline silicon nanowire core issubstantially parallel to a <111> plane and substantially free ofdefects.
 8. The method of claim 7, wherein the step of forming thesilicon dioxide sheathed silicon nanowire that is substantially free ofdefects further comprises the step of forming a silicon dioxide sheathedsilicon nanowire that is substantially free of twinning, substantiallyfree of high order grain boundaries, and substantially free of stackingfaults.
 9. A method of preparing a nanostructure, comprising the step offorming a plurality of substantially monodisperse nanospheres.
 10. Themethod of claim 9, wherein the step of forming the plurality ofnanospheres comprises the step of forming a plurality of substantiallymonodisperse metal nanospheres.
 11. The method of claim 10, wherein thestep of forming the metal nanosphere, comprises the step of forming themetal nanosphere where the metal is selected from the group consistingof: tin, chromium, iron, nickel, silver, titanium, cobalt, zinc,platinum, palladium, osmium, gold, lead, iridium, molybdenum, vanadium,and aluminum.
 12. The method of claim 9, wherein the step of forming theplurality of nanospheres, comprises the step of forming a plurality ofsubstantially monodisperse metal oxide nanospheres.
 13. The method ofclaim 12, wherein the step of forming the metal oxide nanospherescomprises the step of forming a metal oxide nanospheres, wherein themetal oxide is selected from the group consisting of: tin dioxide,chromia, iron oxide nickel oxide, silver oxide, titanium oxide, cobaltoxide, zinc oxide, platinum oxide, palladium oxide, vanadium oxide,molybdenum oxide, and lead oxide.
 14. The method of claim 12, whereinthe step of forming the plurality of substantially monodisperse metaloxide nanospheres, includes the step of forming a plurality ofsubstantially disperse tin dioxide nanospheres.
 15. The method of claim9, wherein the step of forming the plurality of nanospheres, includesthe step of forming a plurality of substantially monodisperse metalloidoxide nanospheres.
 16. The method of claim 15, wherein the step offorming the plurality of substantially monodisperse metalloid oxidenanospheres, includes a step of forming a plurality of substantiallymonodisperse metalloid oxide nanospheres, wherein the metalloid oxide issilicon dioxide.
 17. The method of claim 16, wherein the step of formingthe plurality of substantially monodisperse metalloid oxide nanospheres,wherein the metalloid oxide is silicon dioxide comprises the step offorming an amorphous silicon dioxide nanosphere.
 18. The method of claim16, wherein the step of forming the plurality of substantiallymonodisperse metalloid oxide nanospheres, wherein the metalloid oxide issilicon dioxide comprises the step of forming a plurality ofsubstantially disperse metalloid oxide nanospheres with a diameter rangeof about 8 nanometers to about 45 nanometers.
 19. The method of claim 9,wherein the step of forming the nanosphere, further comprises the stepof forming a nanosphere under thermal conditions.
 20. The method ofclaim 9, wherein the step of forming a nanosphere, further includes thestep of forming a nanosphere under non-catalytic conditions.
 21. Amethod of fabricating catalytic nanostructures, comprising the step ofmetallizing a nanosphere.
 22. The method of claim 21, wherein the stepof metallizing the nanosphere, includes the step of producing at least agram of nanospheres.
 23. The method of claim 21, wherein the step ofmetallizing the nanosphere, includes the step of metallizing a metalnanosphere.
 24. The method of claim 22, wherein the step of metallizingthe metal nanosphere, includes the step of metallizing a metalnanosphere, wherein the metal is selected from the group consisting of:tin, chromium, iron, nickel, silver, titanium, cobalt, zinc, platinum,palladium, osmium, gold, lead, iridium, molybdenum, vanadium, andaluminum.
 25. The method of claim 21, wherein the step of metallizingthe nanosphere, includes the step of metallizing a metalloid oxidenanosphere, wherein the metalloid oxide is silicon dioxide.
 26. Themethod of claim 21, wherein the step of metallizing the nanosphere,includes the step of metallizing a metal oxide nanosphere.
 27. Themethod of claim 12, wherein the step of metallizing the metal oxidenanosphere, includes the step of metallizing a metal oxide nanosphere,wherein the metal oxide is selected from the group consisting of: tindioxide, tin dioxide, chromia, iron oxide nickel oxide, silver oxide,titanium oxide, cobalt oxide, zinc oxide, platinum oxide, palladiumoxide, vanadium oxide, molybdenum oxide, and lead oxide.
 28. The methodof claim 26, wherein the step of metallizing the metal oxide nanosphere,includes the step of metallizing a metal oxide nanosphere, wherein themetal oxide is tin dioxide.
 29. The method of claim 21 wherein the stepof metallizing the nanosphere, includes metallizing a nanosphere with asecond metal.
 30. The method of claim 26, wherein the step ofmetallizing the nanosphere with the second metal, includes the step ofmetallizing a nanosphere with a second metal selected from the groupconsisting of: copper, tin, and aluminum.
 31. A nanostructure,comprising a metal nanowire.
 32. The nanostructure of claim 31, whereinthe metal nanowire comprises a metal wherein the metal is selected fromthe group consisting of: chromium, iron, nickel, silver, titanium,cobalt, zinc, platinum, palladium, osmium, gold, lead, iridium,molybdenum, vanadium, and aluminum.
 33. The nanostructure of claim 31,wherein the metal nanowire comprises a metal oxide nanowire, wherein themetal oxide is selected from the group consisting of: tin dioxide,chromia, iron oxide nickel oxide, silver oxide, titanium oxide, cobaltoxide, zinc oxide, platinum oxide, palladium oxide, vanadium oxide,molybdenum oxide, lead oxide.
 34. The nanostructure of claim 33, whereinthe metal oxide nanowire is a tin dioxide nanowire.
 35. A nanostructure,comprising a metalloid nanowire.
 36. The nanostructure of claim 35,wherein the metalloid nanowire includes a silicon dioxide sheathedcrystalline silicon nanowire, where the axis of the crystalline siliconnanowire core is substantially parallel to a <111> plane andsubstantially free of defects.
 37. A nanostructure, comprising a metalnanosphere.
 38. The nanostructure of claim 37, including a plurality ofsubstantially monodisperse metal nanospheres.
 39. The nanostructure ofclaim 37, wherein the metal is selected from the group consisting of:chromium, iron, nickel, silver, titanium, cobalt, zinc, platinum,palladium, osmium, gold, lead, iridium, molybdenum, vanadium, andaluminum.
 40. The nanostructure of claim 37, wherein the metalnanosphere includes a metal oxide nanosphere, wherein the metal oxide isselected from the group consisting of: tin dioxide, chromia, iron oxidenickel oxide, silver oxide, titanium oxide, cobalt oxide, zinc oxide,platinum oxide, palladium oxide, vanadium oxide, molybdenum oxide, andlead oxide.
 41. The nanostructure of claim 40, wherein the metalnanosphere is a tin dioxide nanosphere.
 42. A nanostructure, comprisingsilicon dioxide nanosphere.
 43. The nanostructure of claim 42, whereinthe silicon dioxide nanosphere has a diameter from about 8 to about 45nanometers.
 44. The nanostructure of claim 42, wherein the silicondioxide nanosphere is metallized with 3 weight percent copper.
 45. Amethod of metallizing a nanostructure, comprising the steps of: forminga nanosphere; metallizing the nanosphere with a metal; and forming ametallized nanosphere that has been metallized with the metal.
 46. Themethod of claim 45, wherein the step of metallizing the nanosphere withthe metal, includes metallizing a nanosphere with copper.
 47. The methodof claim 45, wherein the step of forming the metallized nanosphere,includes the step of forming a metallized copper nanosphere that hasbeen metallized with about 3 weight percent copper.
 48. The method ofclaim 45, wherein the step of metallizing the nanosphere with a metal,includes the step of metallizing a nanosphere with a metal selected fromthe group consisting of: copper, tin, aluminum, silver, platinum,palladium, iron, cobalt, and nickel.
 49. The method of claim 45, whereinthe step of forming the metallized nanosphere, includes the step offorming a metallized metal nanosphere, wherein the metal is selectedfrom the group consisting of: copper, tin, aluminum, silver, platinum,palladium, iron, cobalt, and nickel.
 50. The method of claim 45, whereinforming the nanosphere includes the step of forming a nanosphere underthermal conditions.
 51. The method of claim 50, wherein the step offorming the nanowire under thermal conditions comprises the step offorming a nanowire in the temperature range of about 800° C. to about1500° C.
 52. The method of claim 45, wherein forming the nanosphereincludes the step of forming a nanosphere under non-catalyticconditions.
 53. A method of dehydrogenating ethanol, comprising thesteps of: introducing gaseous ethanol to 3 weight percent coppermetallized silicon dioxide nanosphere; and producing at least 6 percentconversion/mg copper for the selective dehydrogenation of ethanol toacetaldehyde.